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Incredibly, the new M3 sedan and M4 coupe boast identical specifications, with the four-door weighing a scant 10 lb (4.5 kg) more than the two-door.

Lighter, stronger BMW M3/M4 surpass predecessors

2014-07-15
Dan Carney

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For a car with the performance expectations of BMW’s M3 sport sedan, and, under the company’s new nomenclature, its two-door sibling, the M4, we could expect the company to resort to all manner of complex, sophisticated technology to surpass the outgoing model.

And BMW has. But what underpins the success of the new M3/M4 is a focus on solidifying the cars’ foundation with stellar fundamentals. That means an astonishingly rigid unibody, a result achieved through a combination of the advanced—carbon fiber roof on both models—and the simple—rigid-mounted rear suspension subframe.

It is the same for the powertrain, where engineers focused their attention on cooling the engine to help optimize power density. A lighter transmission, borrowed from the smaller 1M, also trimmed weight.

The most fundamental decision for the new M3/M4 was to decide whether it should be powered by a V8 like the outgoing model, or by a six-cylinder, as M3s had been for time immemorial. Rumor circulated that the company was even considering that staple of modern transportation, a V6, but that wasn’t the case, insisted Albert Biermann, head of development for BMW. “There was never one,” he insisted.

But the range of consideration did include a diesel, in addition to the V8 and the inline-six. Ultimately, the company selected a 425-hp (317-kW), 406-lb·ft (550-N·m) twin-turbo 3.0-L inline six-cylinder gasoline engine that can vault the cars from 0 to 60 mph (97 km/h) in 3.9 s.

Despite the two model designations and the extra set of doors on the M3, the two cars are considered the same because their specifications are identical in every respect save the M3’s additional 10 lb (22 lb) of mass and its 3 mm (0.1 in) higher center of gravity, according to Biermann.

The six-cylinder’s aluminum block is a closed-deck design for the sake of rigidity and employs BMW’s LDS sprayed-on twin-wire arc coating rather than cylinder liners for reduced mass.

The engine’s twin turbochargers feature large-diameter throttles to minimize impediments to air flow, and electric turbo boost controls act faster than vacuum waste gates, helping to keep the turbos spinning for maximum responsiveness. “We keep the turbos up to speed all the time in Sport Plus mode,” Biermann said. “If you put obstacles in the way, you can forget about your turbos.”

While there is no dry-sump oil system, the car uses a baffle inside the cast magnesium wet sump and secondary scavenge pump to ensure reliable oil flow under the heavy g-load conditions of track use.

Minimal transmission

The choice of an inline six means that the M-cars’ engines come off the regular straight-six production line, a change from the unique V8 in the previous car. The consequent effect of that is a cost savings that the company could apply to the use of expensive components like the car’s carbon-fiber driveshaft and carbon-fiber strut tower brace.

“Typically you couldn’t afford to do a carbon-fiber driveshaft in a car like this,” Biermann said. That shaft eliminates 40% of the mass of the previous car’s driveshaft for quicker acceleration.

The engine itself is about 10 kg (22 lb) lighter than the old V8. The smaller ZF six-speed manual gearbox is 72 kg (159 lb) lighter than the old M3’s Getrag, and has better shift quality to boot, said Biermann.

What happened when engineers bolted that transmission to the new engine in a test mule for the first time? It broke, naturally, because it wasn’t rated for so much power. But the team reinforced it, as indicated by analysis, and produced a smaller, lighter, better component. A twin-plate clutch also helps withstand the engine’s substantial torque, while providing light effort and exemplary feel, something lacking in recent M3 clutches.

The company takes pride in continuing to offer a traditional H-pattern manual transmission in its sportiest models, said Biermann. Doing so incurs significant cost, both in terms of developing and manufacturing an additional variant, and because the manual transmission stresses the differential and rear axles more than an automatic does, requiring those components to be sturdier than they’d otherwise need to be, he said.

An automatic rev-matching system for downshifts helps prevent inadvertent rear lockup from insufficient engine rpm. A track drive showed the BMW system is less obvious than those used by Nissan and Chevrolet, with just enough revs to make the match, rather than a few more than are really necessary as those other brands’ seem to demand. This makes the system more invisible when on, as it is much less apparent whether it is providing help.

The seven-speed dual-clutch automated manual transmission features integrated launch control, and it executes no-lift upshifts when the driver uses the shift paddles. Its additional ratio provides a second overdrive gear for improved fuel economy compared to the manual transmission.

The clutch is programmed in concert with the stability control system, so it is opened when the car understeers to prevent the rear wheels from pushing the fronts further off course.

Similarly, the rear differential varies its lock from fully open to full locked, depending on circumstances. The new car’s minimum application of just 10 N·m (7 lb·ft) of pre-load, compared to a minimum of 80-90 N·m (59-66 lb·ft) on the old car is a key contributor to the M-cars’ improved corner turn-in steering response, reported Biermann. Where the old car would initially understeer on turn-in, as the partially locked differential induced understeer, the new differential’s ability to be fully open eliminates that shortcoming.

Conversely, the computer recognizes that when a driver presses the accelerator to the floor and holds it there in a turn that they intend to drift the car and it locks the differential completely.

Race-ready chassis

Meanwhile the carbon-fiber strut tower brace features additional attachment points and contributes to the new cars’ 40,000 N·m (29,500 lb·ft) per degree of torsional stiffness, Biemann said. That compares to the 46,000 N·m (33,900 lb·ft)/degree for the famous BMW M3 GTR V8 race car of 2001, with its full roll cage, he added.

“The effort we put in to this car was to bring up the whole structure of the body-in-white,” Biermann noted. To this rigid foundation, BMW bolted lightweight aluminum control arms, wheel carriers, and axle subframes. The unibody is reinforced from beneath with an aluminum stiffening plate, and the rear axle subframe includes additional mounting points for increased rigidity.

That rear subframe bolts directly to the unibody, without the isolation of rubber bushings to introduce vague steering response. The differential mounts to this subframe with its own bushings to reduce noise introduced to the cabin from the differential with little effect on handling.

Another aspect of preserving handling is the M3/M4’s use of electric power steering. Ditching hydraulic power steering is crucial from a manufacturing perspective, where eliminating hydraulic systems from factories is a priority, Biermann said.

With that in mind, his goal was to make the M-cars’ electric power steering “the best EPS money can buy,” he said. “We wanted to provide an M3 where nobody would complain about the EPS.” Achieving that goal in the M-cars was reached through the combination of eliminating external sources of compliance with an extremely rigid platform and through careful programming of the control algorithms for the steering, Biermann said. The use of electric steering also lets BMW offer three different settings for the steering’s assist through the cars’ Comfort, Sport, and Sport Plus settings, which also adjust other factors such as throttle response and transmission shift timing.

The ZF Sachs dampers are monotube design, featuring hollow piston rods. The car rolls on standard 18-inch wheels with Michelin tires, while 19-in wheels are available to provide the necessary clearance for the M-Sport-branded carbon ceramic brakes.

Michelin’s philosophy in developing the BMW’s tires was to press incremental improvements on every front to yield a large overall increase in tire performance, explained Michelin Director of High Performance Tires, Oscar Perada. “It is not just one little widget we improved,” he said. “it is advances on multiple fronts,”

The goal is to manage the contact patch and its contact with the pavement. Perada likens controlling that patch from the tire’s mounting point on the wheel rim as guiding a marionette on strings from a distance.

Advanced computer modeling of the contact patch’s position lets Michelin optimize other aspects, such as the rubber compound, the elastomers used, and the tire’s architecture so that the tires provide high performance with fewer of the typical tradeoffs in tire life, cold-weather traction, and tire noise, he said.